compiletest: tests using revisions treat files with just `//~ ERROR` has having no errors

[pnkfelix]

The revision system lets compiletest encode different sets of errors for different invocations of the compiler.

and compiletest normally attempts to catch cases where you left out expected error messages by saying "if you have any //~ ERROR ... annotation in your file, then the set of such annotations needs to match the same set of errors that are actually emitted.

However, if you combine these features, then for each revision (call it rev) in the revisions list, at least one of your errors currently need to be written with the revision system in mind, e.g. //[rev]~ ERROR ...

If none of the error annotations include a revision (i.e., if they are all //~ ERROR ...), then compiletest thinks that the test has no error annotations, and thus treats it as a case where the compiler's error output is blindly accepted.

That seems bad.

[memoryruins]

cc #55596

[varkor]

This is surely no longer the case, because ERROR and WARN annotations are now mandatory?

[tmiasko]

With annotation for warnings and errors being mandatory #66213, and additionally //~ ERROR annotations applying to all revisions #66524, this should be non-issue.